Switching time shortening dopants for liquid crystalline mixtures

ABSTRACT

Compounds of the general formula C m  H 2m+2 , wherein m signifies 10 to 35, are added to liquid crystalline nematic substances to shorten the switching time of the substances when used in opto-electronic components.

This application is a continuation of application Ser. No. 810,467, filed Dec. 18, 1985, now abandoned.

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT

The invention relates to switching-time-shortening dopants for nematic mixtures, which can be utilized in opto-electronic components (displays) for the electronically controlled modulation of transmitted or reflected light, as well as for the rendition of numbers, symbols and images.

Nematic crystalline liquids can be used in opto-electronic components (displays) G. Meiner, E. Sackmann, J. G. Grabmaier: Applications of Liquid Crystals, Berlin, New York, London 1975.

In order to be suitable for displays, liquid crystals have to fulfill a multitude of requirements, among which there are also included very short switching times which, among other, can also be attained by low viscosities. As viscosity-reducing admixtures dialkylbiphenyl derivatives have become known up to now. J. Krause, R. Steinstraesser, L. Pohl, F. del Pino, G. Weber, DE-OS No. 2,548,360. The disadvantages in the utilization of the biphenyl derivatives are the lowering of the clarification points which have to be increased again by adding other substances. Furthermore, the admixture of approximately 25 weight parts of biphenyl derivatives is required in order to reduce the viscosity by one half, whereby the clarification point is lowered by almost 20° C.

The object of the invention is to provide liquid crystalline mixtures of short switching times for opto-electronic components.

The object of the invention are substances which reduce the rotational viscosity of nematic crystalline liquids and thus lead to fast switching times and which, furthermore, excell in a high effectiveness and are simple to produce.

SUMMARY OF THE INVENTION

It has been found that by the admixture of 0.5 to 30% of one or several compounds of the general formula

    C.sub.m H.sub.2m+2                                         (I)

wherein m=10 to 35, the electro-optical switching time of liquid crystalline nematic substances is considerably reduced.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereonafter, the invention will be explained in greater detail by means of examples.

EXAMPLE 1

The subsequently mentioned mixtures are used in an opto-electronic cell of the Schadt-Helfrich type (twisted nematic structure, TNP cell). The cell consists of two glass plates which, by means of distance spacers, are fixed at a distance of 7 to 25 μm and which are provided on the inside with stannic dioxide. Furthermore, the inside is pre-treated according to conventional methods such that the liquid crystal located between the plates orients itself parallel to the plates, the preferred direction on the two plates, however, including an angle of 90°, so that a twisted nematic structure results, which is arranged between the crossed or parallel polarizers.

By turning the electrical field on and off, strong changes of the light-transmitting capability of the cell are attained.

Into the Schadt-Helfrich cell mixtures of the following compositions are introduced:

Basic component:

4-n-pentyl-cyclohexanecarboxylic acid-4'-cyanophenylester

Dopant:

n-eicosane (n-C₂₀ H₄₂)

The characteristics of mixtures of the basic component with the dopant are summarized in the following table (test frequency 500 Hz, test temperature 42.5° C., thickness of layer 10 μm).

The symbols in the tables have the following meanings:

U_(o) --voltage for an intensity change of 10%

t_(E50) --time from applying the electrical field until an intensity change of 50% is attained

t_(A50) --time from turning off the electrical field until an intensity change of 50% is attained

γ--rotational viscosity

υNI--clarification temperature

υKN--transformation temperature, crystalline-nematic

ΔE_(E) --activation energy for the switching-on times

    ______________________________________                                         mol % Dopant                                                                               0       2          8     10                                        ______________________________________                                         U.sub.o /V  1.10    1.12       1.17  1.18                                      t.sub.E50 (U = 2U.sub.o)/ms                                                                169     122        69    43                                        t.sub.E50 (U = 5V)/ms                                                                      26      19         12    9                                         t.sub.A50 /ms                                                                              75      47         19    11                                        γ/cP  82      41         15    11                                        νNI/°C.                                                                          79      75-76      65-68 61-65                                     νKN/°C.                                                                          47      46.5       46    45.5                                      ______________________________________                                    

The electro-optical Schadt-Helfrich type cell (TNP-cell) described in Example 1 is operated in the further Examples 2 to 7 with basic substance mixtures, to each of which 4% of a compound in the general formula (I) has been added. (Measurement of the switching times at twice the threshold voltage U_(o))

In the examples 2-6 the basic substance mixture A consists of

    ______________________________________                                         Mol %                                                                          ______________________________________                                         21.2                                                                                   ##STR1##                                                               28.2                                                                                   ##STR2##                                                               21.2                                                                                   ##STR3##                                                               8.1                                                                                    ##STR4##                                                               3.7                                                                                    ##STR5##                                                               12.0                                                                                   ##STR6##                                                               5.6                                                                                    ##STR7##                                                               ______________________________________                                    

EXAMPLE 2

    ______________________________________                                                            Mixture A + 4 weight %                                                         C.sub.24 H.sub.50                                           Mixture A          ν.sub.NI = 44° C., ΔE.sub.E =               ν.sub.NI = 46° C., ΔE.sub.E = 2.29 kJMol.sup.-1                                   27.7 kJMol.sup.-1                                           ν/°C.                                                                      U.sub.o /V                                                                             t.sub.E50 /ms                                                                           t.sub.A50 /ms                                                                        U.sub.o /V                                                                            t.sub.E50 /ms                                                                        t.sub.A50 /ms                        ______________________________________                                         20    1.34    156      68    1.24   127   51                                   25    1.23    133      50    1.11   86    44                                   30    1.17    106      41    1.01   79    35                                   35    1.06    90       36    0.93   55    30                                   40    0.98    80       32    0.87   48    27                                   45    0.93    72       31    0.80   45    24                                   50    0.88    67       30    0.75   41    22                                   ______________________________________                                         Mixture A + 4 weight % C.sub.20 H.sub.42                                       ν.sub.NI = 44° C.; ΔE.sub.E = 10.7 kJmol.sup.-1                ν/°C.                                                                         U.sub.o /V    t.sub.E50 /ms                                                                          t.sub.A50 /ms                                   ______________________________________                                         20       1.34          142     50                                              25       1.19          121     44                                              30       1.05          114     42                                              35       0.95          103     38                                              40       0.85          100     35                                              45       0.77           96     32                                              50       0.69           92     29                                              ______________________________________                                    

EXAMPLE 3

    ______________________________________                                         Mixture A + 10 mol %                                                            ##STR8##                     (B)                                              Mixture A and 10 mol % B                                                       υ.sub.NI = 56° C.: ΔE.sub.E = 10.4 kJmol.sup.-1           υ/°C.                                                                    U.sub.o /V   t.sub.E50 /ms                                                                          t.sub.A50 /ms                                    ______________________________________                                         20       1.30         140     59                                               25       1.22         130     42                                               30       1.15         107     34                                               35       1.09         98      31                                               40       1.05         89      28                                               45       0.97         82      26                                               50       0.92         74      25                                               ______________________________________                                         Mixture A + 10 mol % B + 4 weight % C.sub.24 H.sub.50                          υ.sub.NI = 47° C.: ΔE.sub.E = 9.4 kJmol.sup.-1            υ/°C.                                                                    U.sub.o /V   t.sub.E50 /ms                                                                          t.sub.A50 /ms                                    ______________________________________                                         20       1.23         118     43                                               25       1.17         110     36                                               30       1.04         103     31                                               35       0.98         97      27                                               40       0.94         92      24                                               45       0.89         87      21                                               50       0.85         85      18                                               ______________________________________                                    

EXAMPLE 4

    ______________________________________                                         Mixture A + 10 mol % B + 10 mol % C                                             ##STR9##                     (C)                                                                 Mixture A + 10 mol % B +                                                       10 mol % C + 4 weight %                                     Mixture A + 10 mol % B +                                                                          C.sub.24 H.sub.50                                           10 mol % C         υ.sub.NI = 60° C.: ΔE.sub.E =                             7.2                                                         υ.sub.NI = 67° C.: ΔE.sub.E = 10.0                                           kJmol.sup.-1                                                υ/°C.                                                                 U.sub.o /V                                                                             t.sub.E50 /ms                                                                          t.sub.A50 /ms                                                                         U.sub.o /V                                                                           t.sub.E50 /ms                                                                         t.sub.A50 /ms                        ______________________________________                                         20    2.01    168     116    2.14  165    122                                  25    1.65    162     98     1.73  160    97                                   30    1.52    155     86     1.49  151    77                                   35    1.43    146     75     1.33  142    65                                   40    1.33    138     67     1.20  135    55                                   45    1.23    130     61     1.09  125    46                                   50    1.16    122     55     0.98  100    38                                   ______________________________________                                    

EXAMPLE 5

    ______________________________________                                         Mixture A + 10 mol % B + 10 mol % D                                             ##STR10##                    (D)                                                                 Mixture A + 10 mol % B +                                                       10 mol % D + 4 weight %                                     Mixture A + 10 mol % B +                                                                          C.sub.24 H.sub.50                                           10 mol % D         υ.sub.NI = 65° C.; ΔE.sub.E =                             16.6                                                        υ.sub.NI = 74° C.: ΔE.sub.E = 19.4                                           kJmol.sup.-1                                                υ/°C.                                                                 U.sub.o /V                                                                              t.sub.E50 /ms                                                                          t.sub.A50 /ms                                                                         U.sub.o /V                                                                           t.sub.E50 /ms                                                                        t.sub.A50 /ms                        ______________________________________                                         25                            1.68  99    50                                   30    1.60     115     64     1.50  83    38                                   35    1.49     101     51     1.40  72    30                                   40    1.42     90      41     1.32  66    24                                   45    1.35     86      33     1.25  60    21                                   50    1.29     71      28     1.18  55    20                                   ______________________________________                                    

EXAMPLE 6

    ______________________________________                                         Mixture A + 10 mol % B + 10 mol % C + 10 mol % D                                                  Mixture A + 10 mol % B +                                                       C + 10 mol % D + 4                                          Mixture A + 10 mol % B +                                                                          weight % C.sub.24 H.sub.50                                  10 mol % C + 10 mol % D                                                                           ν.sub.NI = 65° C.; ΔE.sub.E =               ν.sub.NI = 77° C.; ΔE.sub.E = 12.5 kJmol.sup.-1                                   10.8 kJmol.sup.-1                                           ν/°C.                                                                      U.sub.o /V                                                                             t.sub.E50 /ms                                                                           t.sub.A50 /ms                                                                        U.sub.o /V                                                                            t.sub.E50 /ms                                                                        t.sub.A50 /ms                        ______________________________________                                         20    2.27    125      80    1.78   72    43                                   25    1.99    116      69    1.63   64    37                                   30    1.78    108      59    1.50   59    32                                   35    1.58    101      51    1.38   55    28                                   40    1.46     94      42    1.27   52    26                                   45    1.36     87      36    1.20   48    23                                   50    1.28     82      31    1.12   46    21                                   ______________________________________                                    

EXAMPLE 7

In example 7 the basic substance mixture consists of a biphenyl mixture E 5 (BDH)

    ______________________________________                                                            Mixture E 5 + 4 weight %                                                       C.sub.20 H.sub.42                                           Mixture E 5        ν.sub.NI = 48° C.; ΔE.sub.E =               ν.sub.NI = 50° C.; ΔE.sub.E = 38.1 kJmol.sup.-1                                   26.1 kJmol.sup.-1                                           ν/°C.                                                                      U.sub.o /V                                                                             t.sub.E50 /ms                                                                           t.sub.A50 /ms                                                                        U.sub.o /V                                                                            t.sub.E50 /ms                                                                        t.sub.A50 /ms                        ______________________________________                                         20    1.63    250      126   1.44   200   88                                   25    1.46    206      92    1.38   170   75                                   30    1.30    167      73    1.34   150   67                                   35    1.18    136      55    1.30   136   61                                   40    1.07     99      48    1.26   126   55                                   45    0.98     80      45    1.21   117   49                                   50    0.90     62      41    1.17   105   45                                   ______________________________________                                         Mixture E 5 + 4 weight % C.sub.24 H.sub.50                                     ν.sub.NI = 49° C.; ΔE.sub.E = 23.8 kJmol.sup.-1                ν/°C.                                                                         U.sub.o /V    t.sub.E50 /ms                                                                          t.sub.A50 /ms                                   ______________________________________                                         20       1.57          145     54                                              25       1.40          100     47                                              30       1.29          75      36                                              35       1.23          66      33                                              40       1.17          59      30                                              45       1.12          55      27                                              50       1.07          52      26                                              ______________________________________                                     

What we claim is:
 1. A composition comprising at least one nematic liquid crystalline compound and at least one dopant compound of the general formula C_(m) H_(2m+2), in which m is an integer from 20 to 24, in an amount sufficient to reduce the rotational viscosity of the nematic liquid crystalline compound.
 2. A composition according to claim 1, in which the proportion of the dopant compound is 0.5 to 4% by weight.
 3. A composition according to claim 1, in which the proportion of the dopant compound is up to 10 mol %.
 4. A composition according to claim 1, wherein the dopant compound comprises C₂₀ H₄₂.
 5. A composition according to claim 1, wherein the dopant compound comprises C₂₄ H₅₀. 